Those instructions work on my 505 (at least up to getting the screen for wiping the device). You have to hold down the Mark and Vol+ buttons for quite a while before the wipe screen comes up (~15 seconds). Maybe the filesystem got corrupted (bad hardware, or USB cable yanked or device reset while files were getting copied over) and it thinks it's part way through a firmware update now. Not sure what you can do about it. Out of curiosity, is there a firmware update file on the device now?